Modern printing operations often rely upon high-speed electronic printers that generate printed output on a throughput continuous web (e.g. a paper web) in a single or side-by-side series of printed pages that are (optionally) slit and cut into individual page sheets. The pages are (optionally) merged and directed downstream into a stacker that creates finished stacks for further downstream handling operations—such as binding, folding, inserting, embossing, punching, etc. The finished stacks can be used for a variety of purposes that are clear to those of skill. Sheets can be received in a stack at a speed that creates certain challenges to generating a properly aligned stack—for example, sheets should be decelerated appropriately to arrive at a proper position in the stack, and should be gripped sufficiently while transiting into the stack to avoid slippage that could result in a misaligned page. Sheets may also be subjected to aerodynamic or electrostatic forces that can affect proper entry into the stack.
Many contemporary stackers employ powered elastomeric wheels or belts in combination with a descending support to direct sheets into a stack. Sheets are received by the wheels/belts and driven into the stack in a desired order. As the stack grows, the support descends to make room for the growing stack height. However, high-speed delivery of sheets can tax the capabilities of such an arrangement.
Moreover, many sheets (substrates) are composed of (or include) materials that add challenges to the stacker and its operation. Many stackers have difficulty handling sensitive substrates, difficult media and applications with heavy or sensitive ink coverage. The thickness of the media can also challenge some stackers. Likewise, it is desirable that stackers be able to handle merged or stream folded substrates.